1. Field of the Invention
The published patent application DE 10 2006 015 387 A1 by M. Hering et al. discloses an interferometric measuring device on the basis of white light interferometry, also known as short-coherence interferometry, wherein the wavefronts of the reflected object ray and those of the reflected reference ray are inclined with respect to one another by a specific angular magnitude by means of an inclination device, such that a spatial interferogram can arise as a single-shot data set. By way of example, said angular magnitude is realized here in a greatly modified Linnik interferometer arrangement, which also has features of a Mach-Zender interferometer, by means of a tilting mirror through which light passes only once on the path to detection. In this case, the tilting mirror disposed downstream of the Linnik arrangement, is situated in an infinite beam path for the object light and is still part of the interferometer.
2. Description of the Related Art
By means of the measuring method on the basis of this optical arrangement, one or a plurality of spatial interferograms, also as line stacks on a matrix camera, can be made available completely as single-shot data sets in the time duration of an image recording.
In this approach with the tilting mirror in the infinite beam path, what is particularly advantageous is that the spatial frequency for the centroid wavelength, or the centroid wavenumber, in the spatial interferogram at the output of the interferometer is, to a first approximation, not influenced by the inclination of the object surface in relation to the interferometer. This invariance of the spatial frequency constitutes a great advantage for the evaluation of spatial interferograms.
However, said downstream inclination device described in DE 10 2006 015 387 A1 is complex—including in terms of alignment—and generally very susceptible to undesirable misalignments and, consequently, the interferometer generally also does not exhibit long-term stability. Thus, the signal waveform of a short-coherent spatial interferogram can also change in an unknown manner, which can constitute a considerable disadvantage for the evaluation.
A targeted change in the angular magnitude by means of an inclination device, for example in order to change the spatial frequency for the centroid wavelength in the spatial interferogram in a predetermined manner, can lead to an undesired lateral offset of object wavefront and reference wavefront during detection, which can be compensated for only in a complex manner or cannot be compensated for at all by an alignment in some cases. Thus, the possibility of changing the spatial frequency for the centroid wavelength in the spatial interferogram in a simple manner is greatly restricted in the case of this arrangement. Furthermore, in the case of a sub-optimal alignment of the very complex interferometer, an only partial lateral overlap of object wavefront and reference wavefront can occur permanently, which can constitute a source of measurement errors or can considerably restrict the depth measurement range since the interference effect disappears when there is a lack of superimposition. In particular, this approach does not enable measurement in a larger depth measurement range than that addressed by means of a single spatial interferogram.
In the publication by M. Hering et al. in Applied Optics, vol. 48, number 3, pages 525 to 538 on 20 Jan. 2009, the measured spatial interferograms in FIG. 3 show the potential of this approach in accordance with DE 10 2006 015 387 A1. The interferometric one-shot measuring arrangement presented in FIG. 1 represents an experimental set-up for study purposes and for economic implementation is rather too complex and too bulky and also very complex in respect of alignment. The optical principle necessitates that in this case the technical and structural measures for achieving a high mechanical stability and temperature stability are very complex. Typical measurement results on the basis of this approach had already been presented by M. Hering et al. in 2006 in the Proceedings of SPIE, vol. 6188, 61880E-1 to 61880E-11 in FIG. 7.
Obtaining spatial interferograms for one-shot metrology, for example for measuring distance or profile, by means of an inclination device in the form of a plane tilting mirror in the interferometer, wherein said mirror is, however, always situated outside the object-imaging and also outside the focused reference beam path, that is to say in an infinite beam path, is regarded as the prior art to be taken into consideration here.
In the case of this principle, the inclination of an object surface with respect to the interferometer advantageously does not lead to a change in the spatial frequency for the centroid wavelength, or the centroid wavenumber, in a spatial interferogram. However, the interference contrast can approach the value zero even when the object surface is tilted slightly.
A comparatively high interference contrast in the detected spatial interferogram can be achieved by means of a comparatively large tilting angle range of the object surface only when said object surface is illuminated by means of an optical system with high numeral aperture and with laterally almost diffraction-limited focal points or focused line images.
By contrast, Michelson-type interferometers having a plane reference mirror, wherein the spatial frequency for the centroid wavelength in a spatial interferogram at the output of the interferometer is to be changed by tilting the reference mirror or by tilting the object with respect to the interferometer or the interferometer with respect to the object, for measurement objects having varying and unknown surface inclination, are not of interest here with regard to the evaluation of spatial interferograms. Therefore, approaches on this basis are not regarded as prior art with respect to this invention and, therefore, nor will they be considered further here.
Obtaining spatial interferograms for one-shot metrology by means of lateral shear between object and reference wavefronts at the output of a two-beam interferometer constitutes, in principle, a further possibility of generating spatial interferograms for one-shot metrology, for example for detecting distance. This is because lateral shear can be used in an optical arrangement as a basis for generating interferences of wavefronts inclined with respect to one another. One entirely traditional approach in this regard is a Michelson interferometer arrangement having two roof edge reflectors in order to generate the required lateral shear. This approach with two roof edge reflectors is generally used for wavefront analysis and is well known to those skilled in the art, also see D. Malacara, Optical Shop Testing, John Wiley & Sons Inc., 1992, pages 140-141, FIG. 4.16 and also W. H. Steel, Interferometry, Cambridge University Press, 1967 page 83 last paragraph to top of page 84.
In order to be able to use this interferometer approach with two roof edge reflectors for distance measurement and profile measurement, in the object beam path of the interferometer an additional plane mirror accordingly has to be assigned to the object surface, wherein said plane mirror together with the object surface then forms a roof edge. In this case, the second roof edge reflector is arranged in the reference beam path. This arrangement, with corresponding alignment, yields lateral shear, avoids wavefront inversion, but generally has distinct disadvantages owing to the structural volume in the object beam path, that is to say with regard to accessibility to the object, for example in the case of measurement in interior spaces.
Also known is the approach published by D. Kelsall in 1959 in Proc. Phys. Society, 73, page 470, FIG. 1, with two triple reflectors as end reflectors of a Michelson interferometer. The lateral displacement of a triple reflector likewise generates a lateral shear between object and reference wavefronts at the output of a Michelson interferometer. To the best of our knowledge indeed the use of a triple reflector in the reference beam path of a Michelson interferometer already goes back, however, to F. Twyman and A. Green, also see U.S. Pat. No. 1,565,533, FIG. 6.
Using this interferometer approach as an arrangement for an interferometric sensor for distance measurement, inter alia, in which a plane mirror of the triple reflector in the form of a corner cube is replaced by the object surface, has the effect that it is also necessary to assign a roof edge reflector or two plane mirrors to the object or to the object surface in the object beam path if the undesired wavefront inversion between object radiation and reference radiation is intended to be avoided. However, this enlarges the sensor volume very considerably, which is very disadvantageous for many applications or this totally precludes the use of such a solution. Moreover, in this case there is no invariance of the lateral shear and thus of the inclination of the interfering wavefronts for example in relation to lateral drifting away of the triple reference reflector or else of the object itself. Thus, the spatial frequency for the centroid wavelength in the spatial interferogram can change, which can be very disadvantageous for the evaluation.